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Biological production and decay of the reactive oxygen species (ROS) hydrogen peroxide (H 2 O 2 ) and superoxide (O2    -) likely have significant effects on the cycling of trace metals and carbon in marine systems. In this study, extracellular production rates of H 2 O 2  and O2    -were determined for five species of marine diatoms in the presence and absence of light. Production of both ROS was measured in parallel by suspending cells on filters and measuring the ROS downstream using chemiluminescence probes. In addition, the ability of these organisms to break down O2    -and H 2 O 2  was examined by measuring recovery of O2    -and H 2 O 2  added to the influent medium. O2    -production rates ranged from undetectable to 7.3 × 10 −16  mol cell −1  h −1 , while H 2 O 2  production rates ranged from undetectable to 3.4 × 10 −16  mol cell −1  h −1 . Results suggest that extracellular ROS production occurs through a variety of pathways even amongst organisms of the same genus.  Thalassiosira  spp. produced more O2    -in light than dark, even when the organisms were killed, indicating that O2    -is produced via a passive photochemical process on the cell surface. The ratio of H 2 O 2  to O2    -production rates was consistent with production of H 2 O 2  solely through dismutation of O2    -for  T. oceanica , while  T. pseudonana  made much more H 2 O 2  than O2    -.  T. weissflogii  only produced H 2 O 2  when stressed or killed.  P. tricornutum  cells did not make cell-associated ROS, but did secrete H 2 O 2 -producing substances into the growth medium. In all organisms, recovery rates for killed cultures (94–100% H 2 O 2 ; 10–80% O2    -) were consistently higher than those for live cultures (65–95% H 2 O 2 ; 10–50% O2    -). While recovery rates for killed cultures in H 2 O 2  indicate that nearly all H 2 O 2  was degraded by active cell processes, O2    -decay appeared to occur via a combination of active and passive processes. Overall, this study shows that the rates and pathways for ROS production and decay vary greatly among diatom species, even between those that are closely related, and as a function of light conditions.

Species-Level Variability in Extracellular Production Rates of Reactive Oxygen Species by Diatoms